Melt the butter in a small pot set over a medium heat. Add the onion and cook until tender, about 5 minutes. While the onions cook, set an oven rack 6" below your broiler. Preheat your broiler to high.
When the onions are tender, mix in the flour. Slowly mix in the beer or stock and bring to a simmer. Mix in the cheese, remove from the heat, and stir until the cheese is melted. Stir in the mustard, salt and pepper. Set the bread slices on a baking sheet and broil each side until lightly toasted. Divide and spread the cheese mixture on the bread. Set the bread slices under the broiler a minute or so, or until bubbly on top.
Set the Welsh rarebit on plates and top each with 2 tomato slices. Drizzle with Worcestershire sauce, sprinkle with parsley, and serve.